Organic electroluminescent (organic EL) is a light-emitting diode (LED) in which the emissive layer is a film made by organic compounds which emits light in response to an electric current. The emissive layer of organic compound is sandwiched between two electrodes. Organic EL is applied in flat panel displays due to their high illumination, low weight, ultra-thin profile, self-illumination without back light, low power consumption, wide viewing angle, high contrast, simple fabrication methods and rapid response time.
The first observation of electroluminescence in organic materials were in the early 1950s by Andre Bernanose and co-workers at the Nancy-University in France. Martin Pope and his co-workers at New York University first observed direct current (DC) electroluminescence on a single pure crystal of anthracene and doped with tetracene under vacuum in 1963.
The first diode device was reported by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The device used a two-layer structure with separate hole transporting and electron transporting layers resulted in reduction in operating voltage and improvement of the efficiency, that led to the current era of organic EL research and device production.
Typically organic EL device is composed of layers of organic materials situated between two electrodes, which include a hole transporting layer (HTL), an emitting layer (EML) and an electron transporting layer (ETL). The basic mechanism of organic EL involves the injection of the carrier, transport, recombination of carriers and exciton formed to emit light. When an external voltage is applied to an organic EL device, electrons and holes are injected from a cathode and an anode, respectively, electrons will be injected from a cathode into a LUMO (lowest unoccupied molecular orbital) and holes will be injected from an anode into a HOMO (highest occupied molecular orbital). When the electrons recombine with holes in the emitting layer, excitons are formed and then emit light. When luminescent molecules absorb energy to achieve an excited state, an exciton may either be in a singlet state or a triplet state depending on how the spins of the electron and hole have been combined. 75% of the excitons form by recombination of electrons and holes to achieve a triplet excited state. Decay from triplet states is spin forbidden, thus, a fluorescence electroluminescent device has only 25% internal quantum efficiency. In contrast to fluorescence electroluminescent device, phosphorescent organic EL device make use of spin-orbit interactions to facilitate intersystem crossing between singlet and triplet states, thus obtaining emission from both singlet and triplet states and the internal quantum efficiency of electroluminescent devices from 25% to 100%. The spin-orbit interactions is finished by some heavy atom such as iridium, rhodium, platinum, palladium and the phosphorescent transition may be observed from an excited MLCT (metal to ligand charge transfer) state of organic metallic complexes.
For full-colored flat panel displays in AMOLED or OLED lighting panel the material used for the phosphorescent guest for emitting layer are still unsatisfactory in half-lifetime, efficiency and driving voltage. These metallic complexes still have disadvantages for industrial practice use.
In the present invention, for the purpose to prolong the half-life time and lower driving voltage for phosphorescent guest in emitting layer for organic EL device, we employ an pyridine-substituted fused fluorene derivative skeleton link to iridium metal, then chelate with one or two 2-phenylpyridine group to finish the metallic complexes represented as general formula (1). The iridium complexes show good thermal stability and charge carrier mobility for organic EL device. Some prior-arts of iridium complexes such as U.S. Pat. No. 8,795,850B2, U.S. Pat. No. 8,778,508B2, U.S. Pat. No. 8,722,205B2, U.S. Pat. No. 8,709,615B2. U.S. Pat. No. 8,779,176B2. But there are no prior arts demonstrate a pyridine-substituted fused fluorene derivative skeleton link to iridium complexes used as phosphorescent emitting guest for organic EL device.